Histologic, histochemical and ultrastructural studies of a tuberous xanthoma from a patient with homozygous type II hyperlipoproteinemia showed that vitually all of the lipid was within histiocytic foam cells; no lipid was identified in interstitial regions or in blood vessels. Primitive mesenchymal cells, elongated perivascular and fibroblast-like cells, and lysosome-filled macrophages also were present within the xanthoma, indicating possible stages in the evolution of dermal mesenchymal cells into mature, cholesterol-rich foam cells. Morphologically the lipid was in 4 different forms: large droplets, which were the dominant form, and membrane-bound crystals, concentric lamellar bodies, and ceroid.